While the use of mechanical couplings to form pipe joints is well known, most such couplings are used to join plastic pipe to plastic pipe or to join metal pipe to metal pipe. However, it is generally preferred to join plastic pipe to plastic pipe by heat fusion and to join metal pipe to metal pipe b welding. In recent years, the usage of plastic pipe, mainly polyethylene pipe, to repair, extend or replace underground gas distribution systems has increased substantially. In many instances, this has resulted in the need to connect a new plastic pipe to an existing metal pipe. The feasibility of joining pipes of the same material in field operations, such as underground gas distribution systems, with either mechanical couplings or fused/welded joints is well accepted. However, making a field installation of a fail safe transition from plastic pipe to steel pipe is sometimes difficult in view of the many adverse field conditions encountered.
Several types of transition fittings have been utilized commercially. Some of the mechanical couplers are designed to be used as transition fittings, but are configured such that one end thereof should only be connected to plastic pipe and the other end thereof should only be connected to metal pipe, requiring the user to make an effort to be certain of the orientation of the coupler or run the risk of a faulty joint. Some of the mechanical couplers have been designed as universal fittings, meaning that they could be used to join any combination of metal and plastic pipes. However, it is difficult to field form a mechanically coupled joint with the same degree of strength and integrity which can be achieved in a factory formed joint. Some factory made transition fittings have been designed with a plastic end and a metal end which can be butt welded to a plastic pipe and a metal pipe, respectively. However, the butt fusion welding of the plastic end to the plastic pipe results in a shortening of the combined length. While this may not be a problem where the metal pipe or the plastic pipe is readily moveable, it can represent a serious problem where both the plastic pipe and the metal pipe are restrained and the fitting has been cut to exactly fit the gap between the free end of the plastic pipe and the free end of the metal pipe. The resulting gap between the metal end of the transition fitting and the free end of the metal pipe may be too great& to permit a satisfactory butt weld, thereby necessitating additional coupling means.